1. Field of the Invention
This invention relates to echo cancellers, and more particularly to a technique by which the echo return loss enhancement is improved.
2. Background Description
Prior to the use of satellite communication circuits, the echo generated at the hybrid, in telephone connections that involve both four-wire and two-wire links, was most commonly accommodated by using echo suppressors. These devices either prevented the reflected (echo) signal from returning to the talker or severely restricted the amplitude of this unwanted signal. This was accomplished by either opening the return circuit path or by inserting a fairly high impedance in the circuit to reduce the amplitude of this unwanted signal. Although this is somewhat annoying (because responses from the listener may be blocked) thus preventing an appearance of face to face communication effective communication could be obtained and thus the slight inconveniences introduced by the use of such echo suppressors was tolerated.
But the disturbing influence on conversation which appears to increase with increased round-trip delay required that other techniques be employed to reduce the echo effect in conversation. Thus echo cancellers were introduced and these operate, not by opening the return path, but by developing a replica of the echo signal in a signal processor and providing this replica of the echo at the appropriate time to one input of a summing device in the return or transmit path from the two-wire to four-wire transition or hybrid. If the characteristic and time of presentation of the echo replica is sufficiently exact the summation will cancel the echo in the return path to the talker. Such echo cancelling systems are well known and many different types have been devised. One such device is disclosed in U.S. Pat. No. 3,500,000, granted Mar. 10, 1970, John L. Kelly, Jr. et al, for a "Self-Adaptive Echo Canceller". In one embodiment, a signal-cancelling echo suppressor employs a self-adjusting transversal filter which is supplied with signals incoming to a four-wire to a two-wire junction. The error signals, derived by processing signals in the outgoing path, continuously control the adjustment of the transversal filter so that the filter produces a replica of an undesired echo at its output. The replica signal is then subtracted from the outgoing signals and the differential is used as a new error signal for controlling the transversal filter. Other devices employ digital techniques to perform the desired signal processing and, in a number of cases, step-wise linear encoding of the signal amplitudes is employed in order to derive a pseudo-logrithmic effect. By so doing a companding effect is introduced. Further, the coding error is reduced for low level signals, but increases the coding error for full scale or high level signals. In addition, the signal processing algorithm behavior may introduce noise, which is more detrimental at the low signal levels. There are also a number of the echo cancellers of the prior art which provide echo return loss enhancement for medium and high level signals, but which do not provide the requisite echo return loss enhancement for low level signals. In some telephone networks using echo cancellers, low signal levels are frequently encountered. Echo cancellers should be effective for speech levels below -30 DBMO, which is not always the case.